Your professors may allow books to be used as sources. Research shared in books is more common in some disciplines than others. For example, history and philosophy publish research in books. Look for books considered scholarly. How do you know if the book is scholarly?
Look for
You still need to think about evaluating these sources, like you do with articles. Think DRAMA - Date, Relevance, Authority, Motivation, and Accuracy..
Contains the full texts of thousands of scholarly and reference e-books from several publishers, covering many subject areas in the humanities, sciences, and social sciences.
Multidisciplinary database containing over 500 peer-reviewed journals and 50,000 academic books published by Oxford University Press and other respected academic publishers. Browse the journals and books, and search within Oxford Academics full collection of books and journals.